An Ejector or Jet Pump is a no-moving-parts pump that uses a high-velocity motive fluid passed through a converging nozzle to entrain and pump a secondary fluid. It is essential in marine engineering, where the Bilge ejector (form 1) clears flooded compartments using engine-room seawater pressure as the only power input. The motive jet drops to low pressure across a venturi throat, drags the suction fluid in, then a diffuser recovers pressure to discharge the combined flow. The result is a rugged pump that handles debris, runs dry without damage, and works anywhere you have a pressurised motive supply.
Ejector or Jet Pump Interactive Calculator
Vary motive pressure, throat pressure, nozzle diameter, and entrainment ratio to see jet velocity and pumped flow through a venturi ejector.
Equation Used
The motive nozzle converts pressure drop into jet velocity. Nozzle area times velocity gives motive flow, and the entrainment ratio estimates how much suction flow is dragged into the mixing throat before the diffuser discharges the combined stream.
- Water service with rho = 1000 kg/m3.
- Incompressible Bernoulli nozzle flow.
- Fixed nozzle discharge coefficient Cd = 0.95.
- Motive and throat pressures are absolute pressures.
- Suction fluid density equals motive fluid density, so entrainment ratio applies to volume flow.
How the Ejector or Jet Pump Works
An Ejector or Jet Pump, also called a Steam-siphon pump / bilge ejector in older marine and boiler-room language, works on Bernoulli's principle applied through a venturi. Pressurised motive fluid — water, steam, compressed air — enters the motive nozzle and accelerates as the cross-section converges. By the time it exits the nozzle and crosses the suction chamber, velocity is high enough that static pressure drops well below atmospheric, sometimes to a few inches of mercury absolute. That low pressure draws suction fluid in through the side port. The two streams mix in the throat, then enter a diverging diffuser where velocity converts back to pressure, lifting the combined flow up to the discharge.
The geometry is unforgiving. The motive nozzle exit must sit at a precise standoff from the throat entrance — typically 0.5 to 1.0 throat diameters — and the throat-to-nozzle area ratio drives the entrainment ratio (mass of suction fluid per unit mass of motive fluid). Get the standoff wrong by even 2 mm on a small unit and the jet either impinges on the throat wall or breaks up before mixing, killing suction. Diffuser angle matters too. Open the cone past about 7° included angle and you get flow separation, pressure recovery collapses, and the pump won't lift its rated head.
Failures are almost always erosion or cavitation, not wear. Sand in the motive water sandblasts the nozzle bore oversized, dropping jet velocity and entrainment ratio. Cavitation appears when suction lift exceeds the motive supply's capacity to maintain throat pressure above the suction fluid's vapour pressure — you'll hear it as a gravelly hiss and see pitting on the diffuser inlet. Run an ejector on cold clean water at modest lift and it will outlast the pipe it's bolted to.
Key Components
- Motive Nozzle: Converging nozzle that accelerates the high-pressure motive fluid into a high-velocity jet. Bore tolerance typically ±0.05 mm on small units; oversized bore from erosion drops jet velocity in proportion to the square of bore diameter.
- Suction Chamber: Low-pressure region surrounding the nozzle exit where the suction fluid enters. Chamber pressure typically 0.1 to 0.5 bar absolute when the unit is operating at design point.
- Mixing Throat: Constant-diameter section where motive and suction streams merge and exchange momentum. Length-to-diameter ratio sits between 4 and 7 — too short and mixing is incomplete, too long and friction losses dominate.
- Diffuser: Diverging cone that recovers static pressure from the high-velocity mixed stream. Included angle held to 5–7° to prevent boundary layer separation; pressure recovery typically 60–80% of theoretical.
- Suction Inlet: Side port that admits the fluid being pumped. Sized for low velocity (under 2 m/s) to keep inlet losses from eating into available NPSH.
Where the Ejector or Jet Pump Is Used
Ejectors and jet pumps appear wherever you need to move a fluid but a mechanical pump is unwelcome — abrasive slurries, hazardous vapours, flooded spaces with no power, or vacuum service where contact with rotating seals is the failure mode you're trying to avoid. The same device shows up under different names depending on the trade. Marine engineers call it a Bilge ejector (form 1) when it dewaters a compartment; chemical plant operators call it an eductor; power-station operators call it a Steam-siphon pump / bilge ejector when steam is the motive fluid; HVAC techs call it a vacuum generator when compressed air pulls a vacuum on a chilled-water system.
- Marine: Bilge ejector on Damen workboats and tugs, using fire-main seawater at 7 bar to clear flooded engine rooms at up to 100 m³/h with no electrical input.
- Power Generation: Steam-jet air ejectors on GE and Siemens steam turbines, pulling 25 mmHg vacuum on the main condenser using auxiliary steam.
- Chemical Processing: Croll-Reynolds liquid eductors mixing polymer feedstock at Dow chemical plants, where rotating seals would fail within weeks on the corrosive service.
- Oil and Gas: Schlumberger jet pumps for artificial lift in mature wells, running power fluid down the tubing to lift crude from depths beyond sucker-rod limits.
- HVAC: Spirax Sarco condensate ejectors removing accumulated condensate from steam mains, replacing failure-prone float traps in high-temperature service.
- Wastewater: Penberthy water-driven sump ejectors on municipal lift stations, draining flooded basements during storm events when grid power is out.
The Formula Behind the Ejector or Jet Pump
The single number that matters when sizing an ejector is the entrainment ratio — kilograms of suction fluid lifted per kilogram of motive fluid consumed. It depends on the area ratio between throat and nozzle and on the pressure ratios across the unit. At low motive pressures (1–2 bar) entrainment ratio sits around 0.3–0.5 for a water-water unit — you spend 1 kg of motive water to lift 0.3 kg of bilge water, which is the cost of having no moving parts. At nominal motive pressure (5–7 bar) entrainment climbs to 0.8–1.2. Push above 10 bar and the jet velocity exceeds what the diffuser can decelerate cleanly, cavitation sets in at the suction port, and entrainment plateaus or even drops. The sweet spot for most marine and industrial work is 5–7 bar motive supply and an area ratio (Athroat / Anozzle) of about 4 to 6.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Qs | Suction (entrained) flow rate | m³/s | gpm |
| Qm | Motive flow rate | m³/s | gpm |
| M | Geometry coefficient from area ratio (typically 0.3–1.2) | dimensionless | dimensionless |
| Pm | Motive supply pressure | bar | psi |
| Ps | Suction pressure (absolute) | bar | psi |
| Pd | Discharge pressure | bar | psi |
Worked Example: Ejector or Jet Pump in a textile dye-house sump ejector
You are sizing a water-driven jet pump to clear spent dye liquor from a 600 mm deep floor sump beneath a Thies iMaster H dyeing machine. The mill water main delivers 5 bar at 60 L/min through a 25 mm motive line. Discharge runs 4 m up to a treatment header at 1.5 bar absolute, and the suction lift is 0.6 m of dye liquor at 1.0 bar absolute (essentially atmospheric in the open sump). You want to know how much liquor you'll actually pull at nominal supply, and how the unit behaves if mill pressure sags during peak shift load or surges during off-shift.
Given
- Qm = 60 L/min
- Pm,nominal = 5.0 bar
- Ps = 1.0 bar absolute
- Pd = 1.5 bar absolute
- M = 0.85 dimensionless (area ratio ≈ 5)
Solution
Step 1 — at nominal 5 bar motive supply, compute the pressure ratio under the radical:
Step 2 — solve for nominal suction flow:
Step 3 — at the low end of the typical operating range, mill pressure sags to 3.5 bar during peak shift load. Motive flow scales roughly with √Pm, so Qm drops to about 50 L/min:
That 37% drop in suction flow is roughly what an operator notices as the sump filling visibly faster than it drains. The ejector hasn't failed — the mill main has.
Step 4 — at the high end, off-shift pressure climbs to 7 bar. Motive flow rises to about 71 L/min:
In theory. In practice, throat velocity at 7 bar pushes suction-port pressure close to the vapour pressure of warm dye liquor, and you'll hear the gravelly cavitation hiss that flags the unit is operating outside its safe envelope. Real-world high-end output sits closer to 170 L/min before cavitation knocks it back.
Result
Nominal suction flow is 135 L/min, which clears the 600 mm sump in roughly 4 minutes — fast enough to keep up with a continuous spill and slow enough that the mill's 60 L/min motive supply isn't strained. Across the operating range, low-end pressure sag drops you to 85 L/min and high-end surge theoretically gives 200 L/min but cavitation caps real output at around 170 L/min, so the design sweet spot really is 5 bar. If you measure 90 L/min instead of the predicted 135 L/min at nominal pressure, check three things in order: (1) motive nozzle bore eroded oversize from suspended fibres in the dye liquor — measure it against the OEM dimension and replace if more than 5% over, (2) air leak on the suction line above the liquor level breaking the vacuum, audible as a sucking gurgle at the joint, or (3) discharge head higher than spec because a partly-closed isolation valve downstream is being mistaken for an open one.
When to Use a Ejector or Jet Pump and When Not To
An Ejector or Jet Pump trades efficiency for ruggedness. A centrifugal pump moves more fluid per kilowatt by a factor of three to ten, but it has bearings, seals, and an impeller that all hate the things ejectors shrug off — debris, dry running, vapour, abrasive slurry, no electrical supply. Pick the ejector when those operating conditions dominate, and pick something else when you're trying to minimise running cost on a clean continuous duty.
| Property | Ejector or Jet Pump | Centrifugal Pump | Diaphragm Pump |
|---|---|---|---|
| Moving parts | Zero | Impeller, shaft, seals, bearings | Diaphragm, check valves, drive mechanism |
| Energy efficiency at design point | 10–30% | 60–85% | 40–60% |
| Tolerance to debris and slurry | Excellent — solids pass through | Poor — clogs and erodes impeller | Good — but check valves can foul |
| Dry-run survivability | Indefinite, no damage | Seconds to minutes before seal failure | Minutes — diaphragm overheats |
| Maximum suction lift | 6–8 m practical | 7 m practical (NPSH limited) | 5–6 m |
| Capital cost (10 m³/h class) | $200–$800 | $1,500–$4,000 | $1,000–$3,000 |
| Installed footprint | Inline, 200–400 mm long | Pump + motor skid | Pump + drive + air supply |
| Typical service life | 10+ years on clean fluid | 3–7 years before overhaul | 1–3 years on diaphragm |
Frequently Asked Questions About Ejector or Jet Pump
Yes — same geometry, different motive fluid. A steam-siphon pump uses superheated or saturated steam as the motive jet instead of pressurised water, and the only design changes are nozzle material (stainless or monel to handle the temperature) and a slightly different area ratio because steam expands across the nozzle while water doesn't.
Steam ejectors give much higher entrainment ratios per kg of motive fluid because steam carries far more energy per unit mass than water at the same pressure. That's why steam ejectors dominated marine and power-station service before electric pumps became reliable.
You've hit the cavitation limit. Once throat velocity is high enough that local static pressure drops to the suction fluid's vapour pressure, vapour bubbles form in the throat and choke the entrainment process. Adding more motive pressure just makes the bubbles bigger — it doesn't lift more fluid.
Diagnostic check: if suction flow flattens or drops as you crank up motive pressure, and you can hear a gravelly hiss at the suction port, you're cavitating. Either lower the suction lift, cool the suction fluid, or step up to a larger throat diameter.
Area ratio is the lever that trades flow for head. Low ratios (2–3) give high discharge head but low entrainment — pick this when you're lifting a small amount of fluid against high backpressure. High ratios (6–10) give high entrainment but low head — pick this for moving large volumes against modest backpressure like a flooded sump.
For most general-purpose dewatering and bilge work, area ratio of 4–6 is the right starting point. It gives roughly equal motive and suction flows at typical mill-water pressures.
Yes, and it's common for vacuum generation on pick-and-place tooling and small lab service. Air-driven ejectors give very high vacuum (down to 27 inHg) but poor mass entrainment because air is 800 times less dense than water.
The catch: air-driven units consume a lot of compressed air. A small vacuum generator pulling 30 L/min of suction air typically burns 80–120 L/min of motive air, which gets expensive on a continuous duty. Use them for intermittent work or where the vacuum requirement is the goal, not the bulk flow.
Almost certainly the motive nozzle has eroded oversize. Even mildly abrasive water (mill water, river water, recycled process water) will sandblast the nozzle bore at a rate of roughly 0.05–0.1 mm per year of continuous service. Once bore diameter grows by 5–10%, jet velocity drops because the flow area went up but pressure didn't, and entrainment falls off a cliff.
Pull the nozzle and measure with a pin gauge. If it's more than 5% over OEM diameter, replace it. Stellite or tungsten-carbide nozzles last 5–10× longer than brass or stainless on dirty service if you want to break the cycle.
When motive pressure drops or you shut the unit off, the suction chamber pressure rises to discharge pressure almost instantly. Without a check valve, that pressure pushes backward through the suction line and blows whatever was being pumped back into the source — often atomising it across nearby equipment.
You can skip the check valve only when the source is at atmospheric pressure, open, and at a lower elevation than the ejector, so backflow is harmless. On any closed system, hazardous fluid, or below-grade installation, the check valve is mandatory.
References & Further Reading
- Wikipedia contributors. Injector. Wikipedia
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